1. Field of the Invention
This invention relates to an improved hydrogen-evolution electrode and a method of producing the same. More particularly, the present invention is concerned with an electrode having a coating comprised of an oxide of at least one metal selected from nickel and cobalt, which coating additionally contains chromium and titanium components in specific proportions. The electrode may be used to conduct electrolysis of sodium chloride or water, during which electrolysis evolution of hydrogen occurs on the electrode in an alkaline solution. The electrode not only advantageously exhibits a low hydrogen overvoltage and high stability for a prolonged period of time but also is available at low cost. Also, the present invention is concerned with a method of producing such an improved hydrogen-evolution electrode.
2. Discussion of Related Art
To attain energy cost saving, researches have been made in the electrolysis industry with respect to electrodes. Especially, intensive efforts have been made to develop an active electrode which exhibits a low hydrogen overvoltage, thereby enabling the superfluous consumption of energy by the hydrogen overvoltage to be decreased. Various proposals have been made to provide an active electrode exhibiting a low hydrogen overvoltage for a prolonged period of time. For example, it was proposed to form an active coating on an electrically conductive substrate by applying an electrode catalyst onto the surface of the substrate by an electroplating method, a chemical plating method, a method comprising applying a coating solution followed by a baking of the solution, a melt-spraying method or the like. It was also proposed to further subject the active coating to heat treatment, or to increase the surface area of the coating by leaching out the soluble sacrificial metal component contained in the coating.
As the above-mentioned electrode catalyst for lowering the overvoltage, investigations have been made to use transition metals, noble metals, combinations of these metals, and combinations of these metal components and sacrificial metal components.
Generally, the heretofore proposed electrodes can be classified into those in which a metal, an alloy or a mixture thereof is used as an active material, and those in which a metal oxide, a compound oxide or a mixture of metal oxides is used as an active material.
The electrodes in which a metal, an alloy or a mixture thereof is used as an active material have a fatal inherent disadvantage in that when an electrolysis is continuously effected using the above-mentioned type of electrode as a hydrogen-evolution electrode, the hydrogen overvoltage of the electrode increases with the lapse of time, and the activity of the electrode is lost. Moreover, the electrode of the above type has another disadvantage that, in the electrolysis of a sodium chloride solution according to the ion exchange membrane method using the above-mentioned type of electrode, a metal component of the electrode dissolves out of the electrode by corrosion or the electrode comes into a passive state by oxidation due to the reverse current occurring at the time of stopping of the electrolysis, whereby the life of the electrode becomes short.
As electrodes eliminating the above-mentioned disadvantages of the electrode of which the active material is a metal, an alloy or a mixture thereof, the electrodes in which a metal oxide, a compound oxide or a mixture of metal oxides is used as an active material have been proposed.
For example, there has been proposed a hydrogen-evolution electrode having on its surface a coating comprising an oxide of at least one metal selected from the group consisting of nickel, cobalt and silver, and a process for preparing the same (see U.S. Pat. No. 4,496,453). This electrode in which the above metal oxide is used as an active material can maintain activity for a prolonged period of time as compared with the electrodes in which a metal, an alloy or a mixture thereof is used as an active material. However, it still has a disadvantage that, from the viewpoint of an industrially useful electrode, the period in which the electrode can maintain activity is not sufficient. The reason is that the metal oxide is gradually reduced to a metal due to the continuous hydrogen-evolution reaction, thereby causing the hydrogen overvoltage to increase with the lapse of time as in the case of the electrode in which a metal, an alloy or a mixture thereof is used as an active material.
Further, there has been proposed a hydrogen-evolution electrode produced by applying, according to a melt-spraying method, onto an electrically conductive substrate a coating comprised of a titanium component and an oxide of at least one metal selected from the group consisting of nickel and cobalt, which titanium component is contained in the coating in a proportion of 0.5 to 20% in terms of atomic percent age (see Japanese Patent Application Laid-Open Specification No. 60-26682/1985). This electrode is characterized by incorporation of a titanium component into the coating to prevent the metal oxide from being reduced to a metal. The durability of the electrode is improved to some extent as compared with the conventional electrodes. However, it still has disadvantages that from an industrial point of view, the durability is not sufficient, and that the lowering of the hydrogen overvoltage becomes insufficient with the increase of the Ti content.
U.S. Pat. No. 4,605,484 (one of the inventors is also one of the present inventors) discloses a hydrogen-evolution electrode comprised of an electrically conductive substrate having thereon a coating layer comprising a chromium component and an oxide of at least one metal selected from the group consisting of nickel and cobalt, which chromium component is present in a proportion of 0.5 to 20% in terms of atomic percentage. In the specification of the U.S. patent, there is a description to the effect that in order to prevent the nickel oxide and/or cobalt oxide in the coating layer from being reduced, it is effective to incorporate therein a component selected from the group consisting of chromium, vanadium, titanium, manganese and niobium, preferably chromium or titanium, more preferably chromium. The proposed electrode in which chromium is used for the purpose of preventing the metal oxide from being reduced is considerably improved with respect to lowering of the hydrogen overvoltage and maintenance of the activity of the electrode for a prolonged period of time. However, the improvements are not sufficient. The present inventors have investigated why the activity of the electrode in which chromium is incorporated is decreased when the electrolysis is conducted for a prolonged period of time. As a result, it has been found that if an electrode in which chromium is incorporated for the purpose of preventing the metal oxide from being reduced is used as a hydrogen-evolution electrode in the electrolysis of an aqueous alkaline solution for a prolonged period of time, the chromium is gradually dissolved into the alkaline solution to lower the chromium content in the coating layer, thereby causing the metal oxide to be reduced to a metal and hence the overvoltage to increase with the lapse of time.
As a result of the extensive and intensive studies, the present inventors have unexpectedly found that the rate of dissolution of the chromium into an alkaline solution can be greatly reduced by further incorporating a titanium component into the coating layer containing a chromium component and an oxide of at least one metal selected from the group consisting of nickel and cobalt. This unexpected finding has enabled us to complete the present invention.